Tungsten is one of the materials of choice for several commercial fusion power plant designs, in particular, for divertor targets and the first wall. In maintenance conditions or during a loss of coolant accident, tungsten is expected to reach temperatures at which it readily volatilises as tungsten trioxide, potentially distributing radioactive material and posing a hazard to personnel. The oxidation of tungsten is reported to show an orientation dependence, however, the mechanism by which it occurs is not fully understood, providing an obstacle to the development of tungsten smart alloys that display reduced oxidation. Using DFT+ U simulations, it is shown how key features of the electronic structure of the tungsten–oxygen system change as the tungsten–oxygen ratio evolves. Formation and migration barriers for oxygen in the different tungsten oxides are determined, allowing an assessment of its mobility in the phases observed during the oxidation process. Our results provide a new level of understanding of the sub-stoichiometric Magnéli phases that are observed during the oxidation of tungsten, which are perceived to be composed of WO2- and WO3-like regions.